A number of brine sources exist naturally. For instance, brine sources include brine deposits like the Salar de Atacama in Chile, Silver Peak Nevada, Salar de Uyuni in Bolivia, or the Salar de Hombre Muerte in Argentina. Other common brine sources are geothermal, oilfield, Smackover, and relict hydrothermal brines. These brines, however, have not previously been commercially exploited very well.
Geothermal brines are of particular interest for a variety of reasons. First, geothermal brines provide a source of power due to the fact that hot geothermal pools are stored at high pressure underground, which when released to atmospheric pressure, can provide a flash-steam. The flash-steam can be used, for example, to run a power plant. Additionally, geothermal brines contain useful elements, which can be recovered and utilized for secondary processes. In some geothermal waters and brines, binary processes can be used to heat a second fluid to provide steam for the generation of electricity without the flashing of the geothermal brine.
One problem associated with geothermal brines when utilized for the production of electricity results from scaling and deposition of solids. Silica and other solids that are dissolved within the geothermal brine precipitate out during all stages of brine processing, particularly during the cooling of a geothermal brine, and may eventually result in fouling of the injection wells or processing equipment.
It is known that geothermal brines can include various metal ions, particularly alkali and alkaline earth metals, as well as silica, iron, lead, silver, and zinc, in varying concentrations, depending upon the source of the brine. Recovery of these metals is potentially important to the chemical, pharmaceutical, and electronic industries. Typically, the economical recovery of metals from natural brines, which may vary widely in composition, depends not only on the specific concentration of the desired metal, but also upon the concentrations of interfering ions, particularly silica, calcium, and magnesium, because the presence of the interfering ions will increase recovery costs, as additional steps must be taken to remove the interfering ions. Economical recovery also depends upon the commercial cost and availability of the desired metal already present in the relevant market.
Silica is known to deposit in piping as scale deposits, typically as a result of the cooling of a geothermal brine. Frequently, geothermal brines are near saturation with respect to the silica concentration and upon cooling; deposition occurs because of the lower solubilities at lower temperatures. This is combined with the polymerization of silica and co-precipitation with other species, particularly metals. This is seen in geothermal power stations, and is particularly true for amorphous silica/silicates. Additionally, silica is a known problem in reverse osmosis desalination plants. Thus, removal of silica from low concentration brines may help to eliminate these scale deposits, and thus reduce costs and improve efficiency of facilities that use and process brines.
Known methods for the removal of silica from geothermal brines include the use of a geothermal brine clarifier for the removal and recovery of silica solids that may be precipitated with the use of various seed materials, or the use of compounds that absorb silica, such as magnesium oxide, magnesium hydroxide, or magnesium carbonate. In addition to a less than complete recovery of silicon from brines, prior methods also suffer in that they typically remove ions and compounds other than just silica and silicon containing compounds.
Geothermal brines can be flashed via several processes. There is the conventional method to produce steam for power. There have also been modifications to the conventional dual direct flash evaporation method to include multiple flash evaporation stages. One modification to the conventional dual direct flash method is the crystallizer reactor clarifier process. In the crystallizer reactor clarifier process, a reactor clarifier precipitates components that can cause scaling, such as iron rich amorphous silica, and removes suspended particles from the brines before injection into the flash process. The process also seeds the brine in the flash vessels to reduce scale formation. Thus, when precipitation occurs it is more likely that it will occur on the seed slurry than on the metal surfaces of the flash apparatus. There is also the pH modification process that differs from the crystallizer reactor clarifier process. In the pH modification process, compounds that cause scaling are maintained in solution. By lowering the pH of the brine solution, for example, as low as 3.0, compounds that typically cause scaling on the flash apparatus are maintained in solution. By lowering pH and modifying pressures, the compounds are maintained in solution and scaling is prevented or reduced.
Thus, although conventional methods employed in the processing of ores and brines can remove some of the silica present in silica containing solutions and brines, there exists a need to develop methods that are selective for the removal of silica from brines and other silica containing solutions at high yields to produce treated compositions with reduced silica concentrations. Additionally, once certain components are removed, the geothermal brine compositions may be injected into a geothermal reservoir, such as into the original reservoir. Compositions for improving injectivity of such brines will improve the efficiency of the process, as improved injectivity will reduce the costs and time associated with cleaning the equipment used for injecting such brines and will also increase long-term permeable flow. While current practices at geothermal plants have focused on reduction of scaling on the apparatus associated with the flash process, there is still a need to reduce scaling after the processing of the brine for energy. The current practice at Salton Sea geothermal plants is to clean injection wells on an annual basis. This is a significant expense as there are typically multiple wells (i.e., three wells) to clean out. This is typically done by hydroblasting or acid treatment. After a certain time, typically three years, this is no longer effective and portions of wells must be routed out to remove blockages, which is expensive and time consuming. The routing process can usually be repeated twice before the wells have to be completely replaced. Thus, compositions and processes that would reduce fouling and prolong the time between required cleanings would be of substantial benefit.
Geothermal brines, such as those found in the Salton Sea, typically include various valuable metal ions, such as silver, manganese, and zinc, dissolved therein. For example, the geothermal brines of the Salton Sea include about 500 mg/L of zinc and about 2000 mg/L manganese dissolved therein. Methods for the economical and selective removal and recovery of high purity zinc from the zinc containing solutions, such as geothermal brines, are needed. These methods would produce brines having reduced concentrations of zinc.